1. Field of the Invention
The present invention relates to a printing system that carries out a secondary scan, which shifts a print head relative to a printing medium in a predetermined direction, so as to print an image on the printing medium. The present invention also pertains to a method of printing an image in such a manner and a recording medium on which a program for realizing the method is recorded.
2. Description of the Related Art
Ink jet printers are widely used as a printing system which prints an image processed by a computer or the like. The ink jet printer forms dots and prints an image by causing ink to be spouted from nozzles mounted on a print head to a sheet of paper. The inkjet printer generally carries out both primary scans, which reciprocate the print head relative to the sheet of paper, and secondary scans, which shift the sheet of paper relative to the print head in a secondary scanning direction, so as to print an image. In one known type of the ink jet printers, a plurality of nozzles are arranged at a predetermined pitch in the secondary scanning direction on the print head. This type of ink jet printer with the plurality of nozzles enables plural rows of dots to be printed simultaneously by one primary scan and thereby has the advantage of the high printing speed.
This type of ink jet printer with the print head having the plurality of nozzles, however, had a disadvantage. A distribution of the ink-spouting characteristics of individual nozzles and a distribution of the pitch between adjoining nozzles may cause a deviation of the dot-forming positions. An observable collection of such deviations is called banding. The banding deteriorates the picture quality of a printed image.
Interlace printing has been proposed as the effective technique to prevent the banding as disclosed in, for example, U.S. Pat. No. 4,198,642 and JAPANESE PATENT LAYING-OPEN GAZETTE No. 53-2040. FIG. 18 illustrates an exemplified process of interlace printing. A variety of parameters are used in the following description. In the example of FIG. 18, it is assumed that three nozzles are actually used for formation of dots. A nozzle pitch k [dots] represents an interval between centers of adjoining nozzles on the print head, which is expressed by a pitch in a recorded image (dot pitch w) as the unit. In the example of FIG. 18, k is equal to two. A number of repeated scans s denotes the number of primary scans required to enable each raster line to be filled with dots. In the example of FIG. 18, one primary scan fills each raster line with dots, so that the number of repeated scans s is equal to one. In case that the number of repeated scans s is two or greater, each primary scan intermittently forms dots in the primary scanning direction. L denotes a feeding amount of the sheet of paper in secondary scan and is set equal to three raster lines in the example of FIG. 18.
In the drawing of FIG. 18, circles, each including a number of two figures, represent the recording positions of the respective dots. In the encircled number of two figures, the left-side figure represents the nozzle number and the right-side figure represents the recording sequence (which time of primary scan the dot is recorded).
In the process of interlace printing shown in FIG. 18, the first primary scan causes a second nozzle and a third nozzle to form dots of raster lines, whereas no dots are formed by a first nozzle. After the sheet of paper is fed by the amount corresponding to three raster lines, the second primary scan is carried out to form raster lines with the first through the third nozzles. The step of feeding the sheet of paper by the amount corresponding to three raster lines and the step of carrying out a primary scan to form raster lines are repeated to record an image. The first primary scan does not cause the first nozzle to form any raster line, since the second and the subsequent primary scans do not form an adjoining raster line immediately below the imaginary raster line formed by the first nozzle in the first primary scan.
The interlace printing intermittently forms raster lines in the secondary scanning direction in the above manner to record an image. The interlace printing has an advantage of dispersing the deviation of the dot-forming positions due to the distributions of the nozzle pitch and the ink-spouting characteristics on a resulting recorded image. The interlace printing method accordingly reduces the effects of the distributions of the nozzle pitch and the ink-spouting characteristics and improves the picture quality of the resulting recorded image.
In the example of FIG. 18, at the specific nozzle pitch, each raster line is formed by one primary scan. In the interlace printing, the nozzle pitch k and a number of nozzles Nnz are generally set to be prime to each other. In this case, the amount of feed L is calculated from the number of nozzles Nnz and the number of repeated scans s according to the equation of L=Nnz/s. The interlace printing is realized under such conditions with varying the parameters.
Even in the interlace printing process, when errors occur in the feeding amount of the sheet of paper in secondary scans, banding due to the accumulated errors may appear to deteriorate the picture quality. The recent advance in resolution of the ink jet printers requires further improvement of the picture quality in printed images. Such deterioration of the picture quality is accordingly not negligible. This problem arises not only in the ink jet printers but a variety of other types of printing systems that form dots to print an image.
In the interlace printing, the parameters, such as the number of nozzles Nnz, are specified to satisfy the above conditions. This may cause part of the nozzles mounted on the print head to be not used for printing in the interlace mode. The nozzles that are not used for printing in the interlace mode may be clogged with the remaining ink.